Field of the Invention
The present invention relates to an oblique-viewing objective optical system which is provided with an optical path converting element, and an endoscope for oblique viewing using the same.
Description of the Related Art
In recent years, in image pickup elements such as a CCD (Charged Coupled Devices) and a C-MOS (Complementary Metal Oxide Semiconductor), due to the progress in microfabrication technology, refining of pixels and small-sizing of an element have been advancing. Especially, recently, image pickup elements having extremely fine pixels such as an image pickup element with a pixel pitch of approximately 2 μm to 3 μm have been manufactured. Thus, the image pickup elements in recent years have a large number of pixels and a small size as compared to former image pickup elements.
Moreover, when a lens outer-diameter and an overall length of an optical system are made small-sized, it becomes difficult to make a light ray emerged from the optical system be incident perpendicularly on a light-receiving surface of an image pickup element. In this case, the light ray is incident obliquely (hereinafter, referred to as ‘oblique incidence’) on the light-receiving surface. Therefore, image pickup elements such as a CCD and a C-MOS of recent years have been designed on the premise that an optimum incidence of a light ray on the light-receiving surface is oblique incidence. In such manner, the image pickup elements in recent years have an oblique-incidence characteristic.
By using the image pickup element with a large number of pixels and small-size in an endoscope, it is possible to achieve a high-quality image and thinning of an endoscope diameter. With this, an objective optical system having a high performance and small-size has been sought for an objective optical system for endoscope. An optical system with a high performance is an optical system with a high resolving power, and in which an aberration is corrected favorably.
As an objective optical system for endoscope, an oblique-viewing objective optical system is available. In the oblique viewing objective optical system, a front viewing, a side viewing, and a rear viewing are carried out.
FIG. 1 is an example of a conventional oblique-viewing objective optical system. An oblique-viewing optical system 1 is an oblique-viewing optical system that carries out the side viewing. The oblique-viewing objective optical system 1 includes a front-side lens group 2, a prism 3, and a rear-side lens group 4. In the oblique-viewing objective optical system 1, due to the prism 3, an optical axis of the front-side lens group 2 and an optical axis of the rear-side lens group 4 are orthogonal.
FIG. 2 is another example of the conventional oblique-viewing objective optical system. An oblique-viewing optical system 5 is an oblique-viewing optical system that carries out the front viewing. An oblique-viewing objective optical system 5 includes a front-side lens group 6, a prism 7, and a rear-side lens group 8. In the oblique-viewing objective optical system 5, due to the prism 7, an optical axis of the front-side lens group 6 and an optical axis of the rear-side lens group intersect (but are not orthogonal).
As shown in FIG. 1 and FIG. 2, in the oblique-viewing objective optical system, an optical path converting element with a large path length in glass is disposed in the optical system. Therefore, especially in the oblique-viewing objective optical system, a large space for disposing an optical path converting element such as a prism is necessary. Consequently, in the oblique-viewing objective optical system, an overall length of the optical system becomes long as compared to a direct-viewing objective optical system. Thus, since an oblique-viewing objective optical system tends to be large-sized as compared to a direct-viewing objective optical system, further small-sizing has been sought in the oblique-viewing objective optical system. Oblique-viewing objective optical systems have been disclosed in Japanese Patent Application Laid-open Publication No. Sho 51-62053, Japanese Patent Publication Nos. 3385090, 3574484, 4439184, and 4814746.
An oblique-viewing objective optical system disclosed in Japanese Patent Application Laid-open Publication No. Sho 51-62053 includes a front-group diverging lens system and a rear-group converging lens system. This objective optical system is an optical system that is supposed to be used in an image fiber. Therefore, in this oblique-viewing objective optical system, an arrangement has been made such that a light ray emerged from the oblique-viewing objective optical system can be incident almost perpendicularly with respect to an end-surface of incidence of fiber.
An oblique-viewing objective optical system disclosed in Japanese Patent Publication No. 3385090 includes a first lens group including one negative lens and a second lens group having a positive refractive power. In this oblique-viewing objective optical system, for correcting a chromatic aberration, a glass material with small dispersion (glass material with a large Abbe number) has been used for the negative in the first lens group lens and a prism.
An oblique-viewing objective optical system disclosed in Japanese Patent Publication No. 3574484 includes a front group having a negative focal length and a rear group having a positive focal length.
An oblique-viewing objective optical system disclosed in Japanese Patent Publication No. 4439184 includes a first lens group including a single lens having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. In this oblique-viewing objective optical system, the third lens group includes in order from an object side, a cemented lens of a negative lens and a positive lens. By making such arrangement, telecentricity is secured. In other words, in the oblique-viewing objective optical system disclosed in patent literature 4, an arrangement is made such that a light ray emerged from the oblique-viewing objective optical system is incident almost perpendicularly with respect to a light-receiving surface of a CCD.
An oblique-viewing objective optical system disclosed in Japanese Patent Publication No. 4814746 includes a first group having a negative refractive power and a second group having a positive refractive power.
Incidentally, in an oblique-viewing objective optical system and a direct-viewing objective optical system, the optical system has been held by a frame member. At a time of assembling, the optical system is to be installed by jigs and tools for assembling via the frame member. Moreover, the optical system after assembly is installed on a front-end portion of the endoscope via the frame member. Therefore, it is necessary to secure a length of certain degree for a fitting portion of the frame member.
FIG. 3 is an example of the frame member of the oblique-viewing objective optical system. An oblique-viewing objective optical system 9 is an oblique-viewing objective optical system that carries out side viewing. The oblique-viewing objective optical system 9 includes a front-side lens group 10, a prism 11, and a rear-side lens group 12. In the oblique-viewing objective optical system 9, by reflecting once at the prism 11, an optical axis of the front-side lens group 10 and an optical axis of the rear-side lens group 12 become orthogonal.
In the oblique-viewing objective optical system 9, the front-side lens group 10 and the prism 11 are held by a frame member 13, and the rear-side lens group 12 is held by a frame member 14. Moreover, an image pickup element 15 is held by a frame member 16.
FIG. 4 and FIG. 5 are other examples of a frame member of an oblique-viewing objective optical system. An oblique-viewing objective optical system 17 and an oblique-viewing objective optical system 18 are oblique-viewing objective optical systems that carry out front viewing. In the oblique-viewing objective optical system 17 and the oblique-viewing objective optical system 18, by reflecting twice at a prism, an optical axis of a front-side lens group and an optical axis of a rear-side lens group intersect. Two frame members have been used in the oblique-viewing objective optical system 17 and three frame members have been used in the oblique-viewing objective optical system 18.